This invention involves the preparation of granular raw materials for use in glass manufacture which reduce gaseous emissions from the glass furnace and improve the operation of the grass furnace by promoting degassing of the glass batch. More specifically, this invention teaches the preparation of alkaline earth metal silicates intimately admixed with alkali metal carbonates or hydroxides.
Commercial production of glass conventionally involves feeding granular glass batch into an opening at one end of an elongated melting furnace while withdrawing melted glass through an opening at the opposite end of the furnace. The term "glass batch" refers to the granular raw materials (between about 0.1 mm and about 1 mm in diameter) for the glassmaking operation which have been physically mixed so as to yield the desired chemical composition after being melted into a homogeneous mass. The typical glass batch contains about 2.8 molecular weights of silica for each molecular weight of metal oxide.
Alkaline earth metals, together with alkali metals are ingredients in some glass formulations. The glass used for the production of face plates for color televisions sets contains up to about 10% strontium and up to about 10% barium by weight with lesser amounts of calcium and magnesium. The strontium and barium ions function as X-ray absorbers to prevent X-rays which are produced when the picture is generated from escaping from the TV set.
In producing television face plates, the economics of scale dictate that large continuous glass furnaces produce as much usable glass as possible. However, the quality of the finished face plate is critical; the presence in the glass of a tiny undissolved particle (called a "stone"), a tiny gas bubble (called a "seed"), or inhomogenity (called a "cold glass defect"), makes the face plate unusable and it must be broken up and remelted in the glass furnace. This broken glass, called "cullet", melts easily in the glass furnace and actually improves the glass furnace operation, however, both capacity and energy cost per ton of saleable glass product suffer when the "cullet ratio" in the glass batch is high.
The capacity of any particular glass furnace producing color television face plate glass is a direct function of how quickly the raw materials in the glass batch can be melted into a homogeneous glass that contains a minimum of stones, seeds, and inhomogeniety defects.
Alkaline earth metal carbonates and alkali metal carbonates are conventionally added to glass batches as the raw material source of alkaline earth metals and alkali metals, respectively. These carbonate raw materials release carbon dioxide during the glass-forming process in the glass furnace. The alkaline earth metal carbonates melt at higher temperatures than the alkali metal carbonates. Carbon dioxide release is believed to occur when the liquid phase in the glass furnace reacts chemically with the surfaces of the sand grains; this means that the alkaline earth metal carbonates, being the carbonates with the highest melting temperatures, are the last to enter the liquid phase and release carbon dioxide. They are thus more likely to contribute to "seeds" in the final glass product.
An issue which is likely to impact the glass industry in the future is the build-up in the atmosphere of carbon dioxide and other gases which absorb infrared radiation and which may contribute to global warming, the so-called "greenhouse effect". The glass industry can minimize emission of carbon dioxide by utilizing economically viable glass batch ingredients that melt and react to form a homogeneous glass more readily (thereby reducing the fossil fuel usage per ton of usable glass produced) and by substituting economically feasible replacements for the carbonate raw materials that are now being incorporated into the glass batch.
All of the sodium carbonate consumed by the glass industry in the United States is "natural" soda ash, that is, it was mined as a carbonate mineral and it underwent minimal chemical transformation prior to being sold as a product. In contrast, the barium and strontium carbonates supplied to the glass industry are derived from naturally-occuring sulfate minerals which first undergo chemical transformation to obtain water-soluble barium and strontium salts, then undergo a second chemical transformation as barium and strontium carbonate are precipitated from aqueous solutions of the respective water-soluble salts.
Alkaline earth metal silicates can be prepared in several ways. They can be precipitated from aqueous solutions of soluble alkaline earth metal salts; U.S. Pat. No. 4,612,292 teaches the precipitation of silicates having more than 2 molecular weights of SiO.sub.2 for each molecular weight of alkaline earth metal oxide. They can be formed directly from the naturally-occuring sulfate minerals; the Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, Volume 3, page 915 states that barium sulfate can be reacted with silica to form barium orthosilicate: EQU 2 BaSO.sub.4 +SiO.sub.2 .fwdarw.Ba.sub.2 SiO.sub.4 +2SO.sub.3
which is transformed in hot water to barium metasilicate: EQU Ba.sub.2 SiO.sub.4 +H.sub.2 O.fwdarw.Ba SiO.sub.3 +Ba(OH).sub.2.
A process developed by the U.S. Bureau of Mines produces hydrogen fluoride and calcium metasilicate from fluorosilicic acid (Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition, Volume 10, page 746.)
Alkaline earth metal silicates can be characterized by the ratio of contained silica to contained metal oxide. The "molar ratio" is the number of molecular weights of silica contained in the material for each molecular weight of metal oxide. The term "metasilicate" refers to a molar ratio of 1.0, while "orthosilicate" refers to a molar ratio of 0.5.
Alkaline earth metal silicates having molar ratios below about 2 would not be conventionally considered candidates for use as glass furnace raw materials because of their high melting temperatures. They are primarily amorphous materials that can vary from almost 0% contained silica to almost 100% contained silica. Phase Diagrams for Ceramists, published by the American Ceramic Society, contains phase diagrams for the binary magnesium, calcium, strontium, and barium silicate systems.
The strontium silicate with the lowest melting point (1358.degree. C.) is shown in Phase Diagrams for Ceramists to contain about 1.9 molecular weights of silica for each molecular weight of strontium oxide (53 weight percent silica). Discrete crystalline strontium silicate species are shown for molar ratios of 0.5 (melting point about 1750.degree. C.) and 1.0 (melting point 1580.degree. C.).
The barium silicate with the lowest melting point (1370.degree. C.) is shown in Phase Diagram for Ceramists to contain 2.9 molecular weights of silica for each molecular weight of barium oxide (53 weight percent silica). This explains the teaching in U.S. Pat. No. 4,612,292 that the silicate products that are "admirable glass-formers" should contain more than 2 and preferably more than 3 molecular weights of silica for each molecular weight of metal oxide. Discrete crystalline barium silicates are shown having molar ratios of 0.5 (melting point above 1750.degree. C.); 1 (melting point 1550.degree. C.); 1.5, 1.6, 1.7, and 2.0 (melting points between 1420 and 1450.degree. C.).
Sodium carbonate melts at a temperature of 851.degree. C.; potassium carbonate melts at a temperature of 891.degree. C. As the glass batch is heated in the glass furnace, the alkali metal carbonates melt and begin to react with the surface of the sand grains present in the glass batch. They may also drain away from the other glass batch ingredients causing inhomogeniety in the glass furnace. U.S. Pat. No. 3,817,776 (Granular Free-Flowing Material for Use in the Manufacture of Glass, M. Gringras) teaches the "fixing" of the sodium by pre-reacting sand and sodium hydroxide to form a coating of sodium metasilicate on the sand grains. The sodium metasilicate has a melting point of 1088.degree. C., so it is claimed that the sodium remains "fixed" until the glass batch reaches a higher temperature and a more homogeneous melt is achieved. This patent teaches a heat treatment after sodium hydroxide solution is sprayed onto the sand grains to obtain the desired chemical reaction.